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Physical separation of soil into different soil organic matter (SOM) fractions is widely used to identify organic carbon pools that are differently stabilized and have distinct chemical composition. However, the mechanisms underlying these differences in stability and chemical composition are only partly understood. To provide new insights into the stabilization of different chemical compound classes in physically-separated SOM fractions, we assessed shifts in the biomolecular composition of bulk soils and individual particle size fractions that were incubated in the laboratory for 345 days. After the incubation, also the incubated bulk soil was fractionated. The chemical composition of organic matter in bulk soils and fractions was characterized by C-CPMAS nuclear magnetic resonance spectroscopy and sequential chemical extraction followed by GC/MS measurements. Plant-derived lipids and lignin were abundant in particulate organic matter (POM) fractions of sand-, silt-, and clay-size and the mineral-bound, clay-sized organic matter. These results indicate that recent conceptualizations of SOM stabilization probably understate the contribution of plant-derived organic matter to stable SOM pools. Although our data indicate that inherent recalcitrance could be important in soils with limited aggregation, organo-mineral interactions and aggregation were responsible for long-term SOM stabilization. In particular, we observed consistently higher concentrations of plant-derived lipids in POM fractions that were incubated individually, where aggregates were disrupted, as compared to those incubated as bulk soil, where aggregates stayed intact. This finding emphasizes the importance of aggregation for the stabilization of less 'recalcitrant' biomolecules in the POM fractions. Because also the abundance of lipids and lignin in clay-sized, mineral-associated SOM was substantially influenced by aggregation, the bioavailability of mineral-associated SOM likely increases after the destruction of intact soil structures. [ABSTRACT FROM AUTHOR]

Volcanic ash-derived soils are important globally for their C sequestration potential and because they are at risk of compaction and degradation due to land use change. Poorly or non-crystalline minerals impart enormous capacity for soils to store and stabilize C, but also unusual chemical and physical properties that make quantifying meaningful soil C pools challenging. Here, we optimize a soil physical fractionation method to effectively assess soil organic matter dynamics in volcanic ash soils by first comparing three common methods for Andisols of the same soil series under three land uses. Components of those methods that (1) effectively isolated C pools of different size and turnover and (2) demonstrated potential sensitivity to land use change were then modified for a final, combined method. The isolation of C pools corresponding to fundamental mechanisms of protection within aggregates and organo-mineral control on the stabilization of C, which often function to the extreme in volcanic ash soils, underlie these modifications. Combined application of ultrasonic energy to disrupt aggregates and the removal of light fractions with sequential high density fractionation successfully isolated multiple C pools that ranged in radiocarbon-based turnover time from 7 to 1,011 year in the surface 0-15 cm of mineral soil in an undisturbed, native forest. Soil C accumulates as a result of high, continuous input that cycles through a transitional (century-scale) organo-mineral pool and then either becomes occluded and protected within aggregates (multiple centuries) or enters a continuum of organo-mineral and non-crystalline mineral-dominated pools (from centuries to millennium-scale). Comparison of relative C pool sizes and C isotope signature among soils from native forest, grazed pasture, and managed Eucalyptus plantation revealed the potential for making accurate, direct measurements of soil C change over time with land use and management change or disturbance regime. [ABSTRACT FROM AUTHOR]

Soil organic carbon (OC) storage across regions is influenced by climate and parent materials, which determine soil properties like clay content and mineralogy. Within homogeneous soil regions, land use and management practices are further important controls for soil OC contents and turnover. Here, we studied the impact of study region, land use (forest, grassland), forest management (spruce and beech forest under age-class management, unmanaged beech forest), and grassland management (meadow, mown pasture, unmown pasture) on stocks and turnover (based on ΔC values) of soil OC in density frations of topsoil horizons. Samples were taken from 36 plots in the regions Hainich-Dün (HAI) and the Schwäbische Alb (ALB) in Germany. They were separated into two light fractions (free light fraction (LF1), occluded light fraction (LF2)) and the mineral-associated organic matter (MOM) fraction using sodium polytungstate with a density of 1.6 g cm. Overall most soil OC was stored in the MOM fraction (73%). Soil OC concentrations and stocks in the MOM fraction differed between study regions, probably due to larger amounts of pedogenic Al- and Fe-oxides in the ALB than in the HAI region. Within each region, forest soils stored significantly higher proportions of total OC in the two LF (33±1.9 %) than grassland soils (20±2.3 %). Different management of forests and grasslands affected the C:N ratio of density fractions, but not OC storage. While modelled soil OC turnover in the MOM was longest of all fractions, all fractions had average ΔC values above atmospheric values, suggesting a significant fast-cycling component in all of them. Different from stocks, turnover of OC in the MOM fraction were not affected by study region or contents of pedogenic oxides. Radiocarbon contents in the LF were higher for forest than for grassland sites, indicating faster turnover of OC at grassland sites. However, some of the observed difference could originate from different average lifetimes of roots in forests and grasslands. Applying different lag-times for OC input for forests and grasslands significantly reduced the differences in modelled turnover times. Lower ΔC values of mown pastures than pasture soils in both regions suggest a management effect on soil C turnover in grasslands.We conclude that OC storage in the MOM of topsoil layers is more affected by regional differences in soil texture and mineralogy than by land use and management, while its turnover could not be explained with the studied soil properties. Soil OC storage and turnover in the two LFs is influenced by land use (forest or grassland) and management, but ecosystem specific lag-times have to be considered for modelling OC turnover in these fractions. [ABSTRACT FROM AUTHOR]